The Spanish prime minister, Mariano Rajoy, has summoned the US ambassador to explain the latest revelations to emerge from the files leaked by Edward Snowden, which suggest the National Security Agency tracked more than 60m phone calls in Spain in the space of a month.

Spain's European secretary of state, Íñigo Méndez de Vigo, is meeting James Costos as the White House struggles to contain a growing diplomatic crisis following accusations that the NSA monitored the phones of scores of allies, including the German chancellor, Angela Merkel.

El Mundo newspaper reported on Monday that it had seen an NSA document that showed the US spy agency had intercepted 60.5m phone calls in Spain between 10 December 2012 and 8 January this year.

An NSA graphic, entitled "Spain – last 30 days", reportedly shows the daily flow of phone calls within Spain, and that on one day alone – 11 December 2012 – the NSA monitored more than 3.5m phone calls. It appears that the content of the calls was not monitored but the serial and phone numbers of the handsets used, the locations, sim cards and the duration of the calls were. Emails and other social media were also monitored.

The news comes as a parliamentary delegation from the EU prepares to visit Washington to discuss the scale of US spying on its allies. The EU's civil liberties committee will meet members of Congress to express their concerns over the impact on EU citizens' fundamental right to privacy.

Last week Spain rejected a move by Germany, which wants the EU's 28 member states to sign a "no-spy deal" along the lines of an agreement wanted by Berlin and Paris.

"We'll see once we have more information if we decide to join with what France and Germany have done," Rajoy said at a press conference in Brussels on Friday.

"But these aren't decisions which correspond to the European Union. They are questions related to national security and are the exclusive responsibility of member states. France and Germany have decided to do one thing and the rest of us may decide to do the same, or something else."

The White House and NSA are coming under intense pressure to reveal the extent to which Obama and senior administration officials knew about US surveillance operations targeting the leaders of allied countries.